Conventional materials composed mainly of titanium have been prone to be easily scratched on the surface thereof owing to low hardness, and have insufficient wear resistance. As a result, in the case of using a pure titanium material, for example, for a watchcase, it has been difficult to enable the watchcase to maintain a high quality external appearance for a long duration. Accordingly, various methods of surface hardening a titanium material have been under intense study.
Conventional methods of surface hardening a titanium material include a method of applying an oxidation or nitriding to the surface thereof. However, these methods have drawbacks in that the oxide layer or nitride layer formed thereby was prone to be easily exfoliated as the same was very brittle and had low impact resistance. Although there is another method of applying high hardness chromium plating to the surface of a titanium material, this method entailed a problem of effluent disposal.
In Japanese Patent Application Laid-open No. 2-250951, a method of surface hardening a titanium material has been proposed wherein nickel (Ni), iron (Fe), cobalt (Co), or the like is placed on the surface of the titanium material, and heated to a temperature higher than a eutectic point of the respective metal with titanium (Ti).
However, as a liquid phase emerges in this method, difficulties will be encountered in removing reaction products remaining on the surface of a surface-hardened titanium-base material during post-treatment processing. Furthermore, in the case where the titanium material thus obtained is used for decorative articles (accessories) or for a watchcase to be worn by a user, there has arisen a risk of residual nickel, iron, cobalt, or the like present the metal surface causing metallic allergy to the skin of the user because the skin will come in direct contact with the surface-hardened titanium-base material. Otherwise, in Japanese Patent Application Laid-open No. 56-146875, a method of enhancing the surface hardness and erosion resistance of a titanium material has been proposed wherein the titanium material is immersed in aluminum oxide (Al.sub.2 O.sub.3) powders, heated, and held in an atmosphere such that a hardened oxidized layer and a dense layer of nitrogen in a solid solution state underneath the hardened oxidized layer are formed in the surface of the titanium material.
However, with this method intended to form a hardened oxidized layer on the surface of a titanium material, it is difficult to control the thickness of the hardened titanium oxide layer formed on the surface and the amount of oxygen in solid solution because intense oxidation caused by oxygen in the atmosphere occurs in spite of the presence of the aluminum oxide powders around the titanium material since heating is applied in the atmosphere. Therefore, there has been a risk of exfoliation occurring due to an increase in the thickness of the hardened titanium oxide layer, and brittle degradation of the titanium material due to an increase in the amount of oxygen in solid solution can occur.
In addition, with the method, uneven contact between the aluminum oxide powders and the titanium material resulted due to the use of aluminum oxide powders not less than 50 .mu.m in grain size, causing another problem that the hardened layer was formed in spots on the surface, and became a porous hardened layer which was prone to be easily exfoliated.
Then, in Japanese Patent Application Laid-open No. 63-195258, a method of enhancing the surface hardness of a titanium material has been proposed wherein the titanium material is introduced into a vessel filled up with calcium carbonate (CaCO.sub.3) powders, the vessel is closed after a partial pressure of oxygen is reduced to not higher than 10.sup.-2 atm, and is heated to a temperature in the range of 900 to 1200.degree. C., causing a carburized layer and an oxygen diffused layer to be formed on the surface of the titanium material by maintaining the temperature.
However, with this method wherein a porous calcium oxide (CaO) layer is formed in the surface besides the carburized layer and oxygen diffused layer, the natural metallic color of the titanium material is lost.
Also, since the treatment temperature was set at 900.degree. C. or more, there was a possibility of this method causing in effect growth of crystal grains, resulting in degradation of quality and high surface roughness. Additionally, the method, wherein a gas resulting from thermal decomposition of calcium carbonate powders is utilized, has had other problems, for example, difficulty with producing stable products safely and efficiently on an industrial basis unless meticulous care is exercised to control the amount of calcium carbonate powders fed relative to the amount of titanium material supplied and the construction and pressure resistant design of the vessel used in the process.
The present invention has been developed to solve various problems described in the foregoing, and an object of the invention is to provide a surface-hardened titanium-base material, capable of preventing exfoliation of the surface layer thereof, having uniformly enhanced surface hardness and wear resistance, unsusceptible to scratches, and not prone to cause metallic allergy. It is another object of the invention to provide a method of surface hardening a titanium material to produce the surface-hardened titanium-base material, and still another object of the invention is to provide products using the surface-hardened titanium-base material.